Hydrometallurgical processes for treating exhausted lead batteries form part of the technical know-how of this industrial sector. Experts of this sector and lead manufacturers both totally agree on the advantages of previously desulphurizing the paste to be reduced pyrometallurgically to lead.
This procedure avoids stack SO.sub.2 emission, which in contrast occurs with direct thermal reduction of non-desulphurized paste. The following desulphurization reactions are currently used: EQU PbSO.sub.4 +Na.sub.2 CO.sub.3 .fwdarw.PbCO.sub.3 +Na.sub.2 SO.sub.4 EQU PbSO.sub.4 +2NaOH.fwdarw.Pb(OH).sub.2 +Na.sub.2 SO.sub.4 EQU PbSO.sub.4 +(NH.sub.4).sub.2 CO.sub.3 .fwdarw.PbCO.sub.3 +(NH.sub.4).sub.2 SO.sub.4
The results of the desulphurization are satisfactory to a greater or lesser extent depending on whether sodium carbonate, ammonium carbonate or caustic soda are used for the conversion. After treatment, the desulphurized paste is suitable for pyrometallurgical reduction, although auxiliary additives have to be added to control sulphur emissions and minimize the lead content of the slag, which has to be dumped on a toxic material dump.
The problems of how to dispose of the Na.sub.2 SO.sub.4 or (NH.sub.4).sub.2 SO.sub.4 produced in the reaction remain.
Na.sub.2 SO.sub.4 can be produced at a purity level suitable for its use in the detergents industry, although this market is in decline.
Alternatively, Na.sub.2 SO.sub.4 can be treated in an electrolytic diaphragm plant particularly designed to reproduce in the cathodic compartment an NaOH solution suitable for desulphurization and in the anodic compartment an H.sub.2 SO.sub.4 solution reusable in batteries. (NH.sub.4).sub.2 SO.sub.4 can be reused as fertilizer, although there is a surplus of this product.
Alternatively, (NH.sub.4).sub.2 SO.sub.4 can be made basic with calcium and the resultant ammonia recycled to desulphurization with the addition of CO.sub.2, however a large quantity of chalk formed in the reaction has to be dumped.
For ecological reasons, some of the said methods have found wide application in the industrial pyrometallurgy of lead, although they have been found costly in practice, and many questions exist regarding continuity of demand for the by-products obtained and the availability of dumps for the slag obtained from pyrometallurgical processes.
The situation is even more complicated for the integrated lead recovery cycle based on hydrometallurgy and electrochemistry. The basic problem of this integrated cycle is that of solubilizing all the lead components of the paste to form a solution suitable for electrolysis in an electrochemical cell to obtain lead.
Numerous processes based on intense experimental work have been proposed to solve this problem.
The following list summarizes the main existing processes:
Gaumann in U.S. Pat. No. 4,107,007 of 1978 leaches the paste with an alkaline hydroxide solution containing added molasses or sugar: the oxide and sulphate pass into solution and go to alkaline electrolysis.
The quality of the lead obtained does not however satisfy market quality requirements.
Elmore in U.S. Pat. No. 4,118,219 of 1978, who converts the sulphate with ammonium carbonate, indicates a series of reducing agents such as formaldehyde, H.sub.2 O.sub.2 and metallic Pb to reduce the PbO.sub.2 of the paste: the object of this process is to obtain lead compounds reusable in the battery sector. The process has never been applied.
Prengaman in U.S. Pat. No. 4,229,271 of 1980 treats the aqueous paste suspension with SO.sub.2 (or alternatively with Na.sub.2 SO.sub.3, NaHSO.sub.3 or NH.sub.4 HSO.sub.3) during desulphurization to reduce the PbO.sub.2 ; he then leaches the product obtained with fluoboric acid and feeds the Pb fluoborate to electrolysis with insoluble anodes for oxygen development.
Ducati in U.S. Pat. No. 4,460,442 of 1984 treats the paste at 100.degree.-120.degree. C. with a concentrated alkali solution to obtain a red lead precipitate which is completely soluble in a hot concentrated HBF.sub.4 or H.sub.2 SiF.sub.6 solution in the presence of metallic Pb. This solution is then fed to electrolysis with insoluble anodes for oxygen development.
Olper in U.S. Pat. No. 4,927,510 of 1990 desulphurizes the paste with NaOH and then leaches with HBF.sub.4 : the insoluble PbO.sub.2 remains in the residue, which is treated with concentrated H.sub.2 SO.sub.4. During the process not only is PbO.sub.2 converted into PbSO.sub.4 (to undergo desulphurization) but all residual organic substances in the paste are eliminated.
Again in this case the final object of the process is to solubilize the PbO.sub.2 to form an electrolyte for electrochemical recovery using insoluble anodes for oxygen development. M.A. Industries in U.S. patent application Ser. No. 850,278 of 1991 desulphurizes the paste with (NH.sub.4).sub.2 CO.sub.3 and then leaches with the fluoboric electrolyte with an added titanium (or alternatively vanadium, cerium or iron) salt which because of its variable valency is able to form with the lead the redox pair necessary for reducing the PbO.sub.2. However the presence of a redox pair in solution interferes negatively with the lead electroextraction.
The ammonium carbonate produced is causticized for regeneration: during the process a large quantity of chalk forms for disposal.
None of the indicated processes is therefore free from drawbacks, namely:
The excessive cost of the reagents used for desulphurization and for reducing the PbO.sub.2.
The progressive accumulation of alkaline metals in the fluoboric (or fluosilicic) electrolyte and the consequent difficulty of purifying it.
The generation of by-products which are often unusable and polluting, and hence have to be dumped.
The low Pb extraction yield, which in industrial practice is between 90% and 95%.
The high energy consumption of the anodic oxygen development reaction.
The need to use anodes which are insoluble, or of very high cost, or of uncertain life.